This invention relates to a process for fabricating channel recess structures in silicon wafers having variable cross-sectional areas, and more particularly to a single side, two step anisotropic etching process which uses etch mask patterns that cause controlled undercutting by the anisotropic etchant to produce channel recesses having predetermined variable cross-sectional areas useful, for example, in the production of ink jet printheads.
In the semiconductor industry, it is frequently desirable to generate large recesses or holes in association with relatively shallow recesses, which may or may not interconnect. For example, an ink jet printhead may be made of a silicon channel plate and a heater plate. Each channel plate has a relatively large ink reservoir with an open bottom, such as a recess etched entirely through the silicon substrate or wafer, and a set of parallel, shallow, elongated channel recesses. One end of the channel recesses are placed into communication with the reservoir and the other ends are opened to serve as droplet ejecting nozzles. When aligned and bonded to a heater plate containing selectively addressable heating elements, the recesses in the channel plate become the ink reservoir and ink flow-directing channels, as described more thoroughly in U.S. Pat. No. Re. 32,572 to Hawkins et al. As recognized by the Hawkins reference, a fundamental physical limitation of anisotropic etching of silicon or orientation dependent etching (ODE), as it is sometimes referred to, is that {111} crystal planes etch very slowly, while all other crystal planes etch rapidly. Consequently, only rectangles can be etched in ( 100) silicon material or wafers with a high degree of precision.
U.S. Pat. No. 4,774,530 to Hawkins discloses a two-part ink jet printhead comprising a mated channel plate and a heater plate, which sandwiches a thick film insulative layer that was previously deposited on the heater plate and patterned to provide an ink bypass recess for ink flow from the reservoir to the channels and recesses or pits over each heating element for placement of the heating elements in pits to prevent the vapor bubbles from blowing out the nozzles and causing ingestion of air. This is a typical ink jet printhead configuration and is discussed later with respect to FIG. 1
U.S. Pat. No. 4,863,560 to Hawkins discloses a three dimensional silicon structure, such as an ink jet printhead, fabricated from (100) silicon wafers by a single side, multiple step ODE etching process. All etching masks are formed one on top of the other prior to the initiation of etching, with the coarsest mask formed last and used first. Once the coarse anisotropic etching is completed, the coarse etch mask is removed and the finer anisotropic etching is done. The same anisotropic etchant of KOH is used for both coarse and fine etching. Since the coarse etch mask material is silicon nitride and the fine etch mask material is thermally grown silicon dioxide, erosion of the silicon dioxide in KOH must be taken into account and the silicon dioxide mask made appropriately thick.
U.S. Pat. No. 5,096,535 to Hawkins et al. discloses the fabrication of a printhead, wherein each of the ink channels is formed by segmenting the channel mask into a series of closely adjacent vias, such that during the subsequent anisotropic etching of the silicon wafer, the thin walls between the segments are eroded away before the completion of the etching step to produce the channels from the connected segments. Thus, mask alignment errors that would cause the channels to be greatly widened when the channel are one long recess are greatly reduced.
As is well known, the geometrical parameters and/or configurations of the ink flow paths in ink jet printheads determine the frequency of the droplet ejection and thus the printing speed. For example, some of the important geometrical parameters are the size of the nozzles relative the cross-sectional areas of the channels, and size of the ink flow area at the entrance to the channel relative to the nozzles, for these dimensions influence capillary refill times from the ink supply in printhead reservoir. However, because of the constraints on the anisotropic etching of silicon, the channels in printheads generally have substantially uniform cross-sectional areas. Therefore, there is a need for more flexibility in the design and fabrication of silicon channel structures in ink jet printheads.